Arousal – An arousal is a brief awakening from sleep that lasts at least 3 seconds. Apnoea and hypopnoea events often cause arousals from sleep because a person will wake briefly to breathe normally again. Most arousals from sleep are brief enough that a person does not remember them. Nonetheless, they are disruptive to sleep and may cause daytime sleepiness or fatigue.

Sleep Efficiency – Sleep efficiency refers to the percentage of time a person sleeps, in relation to the amount of time a person spends in bed. The percentage is calculated by dividing Total Sleep Time by Total Time in bed. Normal sleep efficiency is considered to be 80% or greater. For example, if a person spends 8 hours in bed (from 10 p.m. to 6 a.m), at least 6.4 hours or more should be spent sleeping to achieve an 80% or greater sleep efficiency. Most healthy and young adults have sleep efficiencies above 90%.

Sleep Latency – Sleep latency refers to the amount of time it takes a person to fall to sleep. For example, if a person goes to bed at 10:00 and falls to sleep at 10:15, the sleep latency is 15 minutes. Normal sleep latency is 5-15 minutes. Sleep latency less than five minutes may suggest some degree of excessive sleepiness. Sleep latency greater than 15 minutes may suggest some difficulty with sleep initiation.

Sleep Architecture – Sleep architecture refers to the structure of sleep. There are four stages of sleep that make up each sleep cycle. Each sleep cycle lasts about 90-120 minutes; thus, a person should have 4-5 cycles of sleep each night.

Stages of Sleep – As stated above, there are four stages of sleep with the following descriptions:

Stage 1 sleep is the first and earliest stage of sleep. Often referred to as “drowsiness” or “pre-sleep”, it is very light sleep. During this stage, individuals usually have an awareness of their surroundings and may not perceive themselves to be asleep. Only about 5-10% of total sleep time is spent in Stage 1 sleep.

Stage 2 sleep is the predominant sleep stage during a normal night of sleep. An individual does not have an awareness of surroundings during this stage and is approaching a deeper state of sleep. About 25-45% of sleep time is spent in this stage.

Stage 3 sleep is deep sleep. During this stage, individuals are more difficult to wake from sleep and if they do wake from this stage, they typically feel drowsy. About 20-25% of sleep is comprised of deep sleep. Older individuals tend to have a lower percentage of deep sleep.

Stage R sleep is REM sleep. Rapid Eye Movement (REM) sleep is otherwise referred to as dream sleep. About 20-30% of sleep time is spent in this stage, with the largest majority of REM occurring late in the night.

REM Latency – REM latency refers to the amount of time it takes a person to achieve REM (or dream) sleep after the person has initially fallen to sleep. As REM is the fifth and final stage of a sleep cycle, normal individuals achieve their first period of REM sleep about 70-110 minutes after falling to sleep.

Oxygen Saturation (SaO2) – Oxygen saturation refers to oxygen levels in the blood. Normal oxygen saturation in a healthy adult is 94-100%. In persons with sleep apnoea or lung disease, oxygen saturation levels are often reduced.

Periodic Limb Movement Index (PLMI) – Periodic Limb Movements are leg jerks during sleep that occur every 5-20 seconds. If the leg jerks are robust, they may cause a person to briefly awaken from sleep. The (PLMI) is a measure of the number of leg jerks each hour of sleep. If leg movements are excessive, treatment with supplements or medication may be indicated.